ABSTRACT

Rare earth element (REE) contents of alkaline rocks from Tahiti have been determined. Their chondrite normalized patterns are characterized by relative enrichment of the light REE, as is common in other alkaline rocks. Additionally, two distinct fine structures are seen, especially in basic rocks: An upward-concave shape of the heavy REE span and an inflection in the light REE span. The author has attempted to explain these fine structures and to derive information on the petrogenetic processes based on calculated models. It is concluded that the upward-concave shape is generated under high pressure conditions corresponding to the deeper part of the uppermost mantle. Furthermore, considerably large amounts of clinopyroxene fractionation are needed to explain both the inflectional feature of the light REE span and the upward-concave shape of the heavy REE span. The most appropriate model for the parental magma for Tahitian rocks is 6 to 10% melting of a garnet peridotite mantle followed later by 60 to 70% of clinopyroxene fractionation at high pressure.